Research on processes influencing the development of affective brain circuits is critical to elucidating the neurobiological substrates of psychiatric disorders. Mechanistic evidence from adults showing a sleep- dependent functional ?disconnect? between brain regions central to adaptive emotion processing (i.e., regulation and expression) suggests that sleep loss is a fundamental target. Similar data in young children, however, do not exist. Early childhood is a sensitive period in the maturation of sleep and emotion processing and also a time when disturbance in both domains is commonly first detected. Further, epidemiological findings reveal that insufficient sleep in childhood is prevalent, associated with concurrent emotional problems, and predicts later mood and attentional disorders. Although our recent experimental findings indicate that acute sleep loss results in non-adaptive emotion processing in young children, the neural systems underlying such sleep-dependent effects are not known. Also, the vast majority of basic research on sleep and affective substrates has utilized sleep deprivation or sleep restriction protocols. We will instead employ sleep extension in chronically sleep-restricted children, a highly translatable approach with significant public health implications. This R21 Exploratory/Developmental research project will examine whether experimental sleep extension in children alters the neural and behavioral mechanisms by which short sleep is a risk factor for emotional/behavioral problems. Children ages 5.0-5.9 years with chronic insufficient sleep (?9 h/night for ?6 months) will be randomized to either a sleep Extension or to an active Control group. Extension group parents will participate in a 1-month individualized behavioral sleep intervention to promote targeted sleep duration improvements before beginning a 2-week sleep Extension schedule (8 week protocol). Brain and behavioral assessments will occur at Baseline and post sleep Extension. These include structural MR, fMRI and in- scanner video of facial emotion expressions during a task eliciting negative and positive emotions, and in- home testing of emotion processing and executive function. The following hypotheses will be tested: 1) Extension compared to Control group children will show decreased negative emotion sensitivity and increased positive emotion sensitivity as measured by fMRI activation of positive/negative affect systems, in-scanner facial emotion expressions during task completion, and in-home behavioral assessments; and 2) Extension compared to Control group children will exhibit increased functional connectivity between regulatory/context- processing systems and positive/negative affect systems during task completion. The proposed research is in line with several strategic goals of the NIMH, and the approach includes measures that span a number of constructs of the Research Domain Criteria Project. The results of this work are critical to developing a mechanistic understanding of sleep ? emotion links during early childhood and will inform early prevention and intervention programs targeting children with emotional/behavioral problems.